1. Field of the Invention
This invention relates generally to a video signal reproducing apparatus, and more particularly is directed to improvements in such apparatus for avoiding skew distortion when reproducing is effected with the magnetic tape or other record medium being advanced at a speed and/or in a direction other than that used for recording.
2. Description of the Prior Art
It is well known to record video signals on a magnetic tape or other record medium by scanning successive parallel tracks on the record medium with one or more transducers energized by the video signals. In effecting such recording of video signals, it has been the usual practice to provide guard bands or unrecorded gaps between the successive parallel tracks so that, when a transducer scans one of the tracks for reproducing the signals recorded therein, such transducer will not also reproduce cross-talk, that is, signals recorded in the adjacent tracks. However, the provision of guard bands between the successive parallel tracks reduces the recording density, that is, the amount of signal information recorded on a unit area of the record medium, and thus does not permit the efficient utilization of the record medium for the recording of video signals.
In order to minimize cross-talk while permitting an increase in the recording density, it has been conventional to use two transducers or heads having air gaps with different azimuth angles for recording and reproducing signals in the next adjacent or alternate tracks, respectively. This is relatively easy to do because apparatus for magnetically recording and/or reproducing video signals frequently includes a rotary guide drum provided with two alternately operative transducers or heads which are diametrically opposed and which can have air gaps with different azimuth angles. The magnetic tape is wrapped helically about a portion of the perimeter of the guide drum and is moved longitudinally while the transducers or heads are rotated, thus causing the heads alternately to scan respective tracks on the tape for recording or reproducing signals therein. In the recording operation of the foregoing helical scan type VTR (video tape recorder), each head effects magnetization of magnetic domains in the magnetic coating on the tape in what would appear to be, if such domains were visible, a series of parallel lines or stripes each having a length as great as the width of the track, and each having an orientation that corresponds to the azimuth angle of the gap of the respective transducer or head. In the reproducing or playback operation of the apparatus, each track is scanned by the transducer or head having its gap aligned with the parallel, but fictitious, lines of that track, from which it follows that the gap of the transducer or head scanning a track for reproducing the video signals recorded therein extends at an angle to the mentioned fictitious lines of the tracks next adjacent to the track being scanned. By reason of the foregoing, if a transducer or head, in scanning a track for reproducing the video signals recorded therein, overlaps a next adjacent track or otherwise reproduces signals recorded in the latter, the well-known azimuth loss will result in attenuation of the cross-talk signal reproduced from the next adjacent track.
When recording color video signals which include luminance and chrominance components, it is known to separate such components and then to frequency modulate a relatively high frequency carrier with the luminance component, while the chrominance component is frequency converted so as to have its frequency band shifted below the frequency band of the frequency-modulated luminance component, whereupon the frequency-modulated luminance component and the frequency converted chrominance component are combined to provide the composite video signals which are recorded in the successive parallel tracks. Since the previously mentioned azimuth loss is generally proportional to the frequency of the signals, the azimuth loss is relatively effective to decrease or eliminate interference due to cross-talk in respect to the relatively high frequency frequency-modulated luminance component. However, interference due to cross-talk from the relatively low frequency or frequency-converted chrominance component is not sufficiently reduced by the use of transducers having different azimuth angles. Thus, when recording color video signals, it has been proposed, for example, as disclosed in detail in U.S. Pat. No. 4,007,482, issued Feb. 8, 1977, and having a common assignee herewith, to reduce or eliminate interference due to cross-talk in respect to a relatively low frequency signal recorded in next adjacent tracks by recording the frequency converted chrominance component or other low frequency signal in such adjacent tracks with different first and second carriers, respectively, which may be distinguished from each other by their respective polarity characteristics. In a particular disclosed embodiment of the foregoing scheme, the first carrier for the frequency-converted chrominance component has its phase unchanged throughout the recording of the video signals in a respective track, while the second carrier for the chrominance component recorded in the next adjacent track has its phase inverted or changed by 180.degree. for successive line intervals in the case of recording NTSC color video signals, or after every two line intervals in the case of recording PAL color video signals. When a head scans a particular track for reproducing the video signals recorded therein, the chrominance component of cross-talk signals from the tracks next adjacent to the scanned track can be conveniently suppressed or eliminated, for example, with the aide of a simple comb filter, by reason of the different polarity or phase characteristics of the carriers with which the chrominance component was recorded in the scanned track and in the tracks next adjacent thereto, respectively.
If, during reproducing operation of the helical scan type VTR, the speed and direction of movement of the tape are equal to the standard tape speed and direction, respectively, for recording, then the scanning path of each rotary head or transducer can be made to accurately coincide with a respective one of the record tracks for properly reproducing the video signals recorded therein, and the above described measures are effective for substantially eliminating cross-talk in respect to signals recorded in the tracks next adjacent the track being scanned.
However, in practical embodiments of the helical scan type VTR, the standard tape speed for recording is conveniently selected in relation to the diametrical size of the guide drum so that the positions at which the horizontal synchronizing pulses of the video signals are recorded in the nearest to each other tracks recorded by the same head are displaced from each other, in the direction along the tracks, by 1/2 of a horizontal period (H), that is, 1/2 of the distance that the rotary head or transducer moves during a horizontal or line period of the video signals. Thus, in the case where odd and even numbered fields of the video signals are respectively recorded by first and second diametrically opposed heads in alternating tracks on the magnetic tape, the positions at which the horizontal synchronizing pulses are recorded in a track containing an odd numbered field, for example, will be spaced or shifted by distances corresponding to 1/2 the horizontal period (H) in respect to the positions at which horizontal synchronizing pulses are recorded in the next adjacent or nearest tracks containing odd fields. By reason of the foregoing, when a reproducing operation is performed with the tape being driven in the direction opposite to that for recording, or with the tape being driven in the same direction as for recording, but at a substantially greater speed, a so-called skew distortion may appear in the reproduced picture.
More particularly, if the tape speed during reproducing is substantially different from that for recording, or if the direction of tape movement during reproducing is different from that during recording, the scanning path of each head may be at a sufficiently large angle to the direction along the recorded tracks so that, in traversing such scanning path, each head will move along, and reproduce video signals from first one and then another of the nearby tracks which have the positions of the recorded horizontal synchronizing pulses offset by 1/2 the horizontal period (H). If such signals reproduced from first one track and then another during a single field interval are demodulated and fed to a television receiver, a disturbance of jump of 1/2H occurs in the continuity of the horizontal synchronizing pulses at the change over of the reproducing head or transducer from one track to the other. During the scanning period in which the automatic frequency control (AFC) circuit of the television receiver absorbs the jump of 1/2H, a skew distortion will appear in the reproduced picture.